In the electronics industry ceramic substrates are used for mounting semi-conductor chips or the like and for printing, depositing by vapor deposition, or otherwise applying the electrical circuitry associated with the chips. As the electronics art progresses, there is an ever increasing need for more electrical circuitry per unit area of the substrate. This, in turn, requires that the conductive paths applied to the substrate to provide the circuitry be of decreasing width so as to occupy less surface area per unit length of the conductive path. Such fine-line circuitry imposes a requirement that the substrate surface to which the circuitry is applied have improved planarity, i.e. minimal surface defects such as pits and voids.
Ceramic substrates for such end use are conventionally made of alumina ceramic though beryllia can be used where very high thermal conductivity is desired, albeit at higher cost. But all such ceramics are very hard and hence surface polishing is difficult and expensive. Further, whereas polishing does impart smoothness, it does not necessarily eliminate surface defects, such as relatively deep pits, which cause difficulty in the attainment of a good fine-line circuitry pattern. The conventional solution to the problem of surface defects is to apply a thin layer of glaze, i.e. a glass, to the surface of the ceramic. However, the application of glaze results in several undesirable characteristics such as reductions in adhesion, thermal conductivity, chemical resistance and electrical insulation between adjacent conductive paths in the circuitry. This is because glass is much inferior to alumina or beryllia ceramic in all these respects.